大家好 (da4 jia1 hao3) hi everyone
Composition: 大 (Big/large) + 家 (House/family) + 好 (Good/ok)
You can think about it like you're asking "Is everyone (in the house) good?"
One reply you can answer, especially if it's the teacher saying this:
老师好 (lao3 shi1 hao3) Hello teacher
Composition: 老师 (teacher) + 好 (good/ok)
Similar to the previous, the word-for-word translation is like asking "Is teacher good?"
If you want to ask the question of 'How are you?' at the start of the question:
你好吗？(ni3 hao3 ma?) How are you?
(There's a grammar thing to 吗 - I encourage you to look it up)
吗 is basically a question particle, often used to end a question. Here, the question could be understood as "You good?" [literal translation]
If I say that "I" is 我 (wo3), then how would you reply to the question?
Side note: not a member, but I like to help others learn Mandarin
